In a conventional microwave heating apparatus adapted for both dielectric heating and radiant energy heating, it has been the general practice to install the microwave source beneath a heating chamber to avoid temperature buildups therein, and therefore an elongated wave guide is required to supply the microwave energy to the heating chamber from an overhead position. This arrangement not only has led to large losses of microwave energy but has also resulted in increased structural complexity and manufacturing cost of the apparatus.
When the microwave energy is supplied to the heating chamber from beneath the heating chamber, the wave guide can be short but in such a case, the microwave energy is reflected or partially absorbed by the container holding the object to be heated and by the supporting means, if any, on which the container rests, with the result that the efficiency of dielectric heating is sacrified.
There also are apparatuses in which a microwave generator is positioned above the heating chamber. In such an arrangement, the efficiency of dielectric heating is satisfactory but because the heat from the radiant heating is directly transmitted to the microwave generator, it is necessary to use a high-capacity cooling fan for the microwave generator. This not only means an increased production cost but, because much heat escapes from the heating chamber, the efficiency of radiant heating is sacrificed.